This invention relates to the field of ion optics and, more particularly, to apparatus for scanning an ion beam over the surface of a workpiece while maintaining a substantially constant angle of incidence.
Ion implantation has become a standard technique for introducing impurity dopants into semiconductor wafers. A beam of ions is generated in a source and is directed with varying degrees of acceleration toward a target wafer. Ion implantation systems typically include an ion source, ion optics for removing undesired ion species and for focusing the beam, means for deflecting the ion beam over the target area, and an end station for mounting and exchanging wafers.
In most cases, the cross-sectional area of the ion beam is substantially smaller than the area of the target wafer. Therefore, relative motion between the wafer surface and the ion beam is necessary in order to distribute the implanted dose. If the target is moved in its own plane and the beam remains stationary, a situation typical of mechanically scanned systems, the angle of incidence of the beam clearly remains constant. In the more usual apparatus, the ion beam is scanned in one or two dimensions over the surface of the workpiece by magnetic or electrostatic deflecting fields. In such cases, the angle of incidence varies with beam deflection angle. As a result, the ion dose per unit area of the workpiece varies as the sine of the deflection angle.
The trend in semiconductor wafers has been toward larger diameters (up to 8 inches) to achieve economies of scale. As wafer dimensions increase, dose variations also increase due to the larger required deflection angle. Alternatively, the length of the scanning apparatus can be increased in order to maintain a constant angle of deflection. Neither alternative is acceptable, since dose uniformity requirements have become more stringent, while clean room floor space is at a premium. Dose variations due to scan angle can be compensated, as described in U.S. Pat. Nos. 4,283,631 and 4,449,051. However, it is more practical to maintain a constant angle of incidence without the necessity for compensation. Furthermore, the channeling effects described hereinafter cannot be easily compensated.
Variations in ion beam incidence angle present problems in connection with channeling effects. It is known that incident ions of a given energy penetrate into the crystal lattice of the target by different distances, depending on their angle of incidence with respect to various crystal planes. Thus, the variations in incident angle for a scanned ion beam result in different penetration depths and a corresponding variation in device characteristics over the surface area of the wafer. Such effects are difficult to compensate. It is customary in ion implantation to tilt a silicon wafer by a few degrees with respect to the incident ion beam to minimize channeling effects. Preferably, the angle should be maintained constant over the surface area of the wafer. Constant angular incidence has been achieved with electron beams using magnetic and electrostatic double deflection scanning, as shown in U.S. Pat. No. 4,101,813 and 4,117,339. Double deflection systems are practical in the case of electron beams due to the typically small electron currents and the small electron mass. One drawback of double deflection systems is that they require a careful synchronization of time-varying electrical inputs to the deflection elements.
Although double deflection systems can, in theory, be applied to ion beam systems, these systems have severe practical limitations. Ion implantation systems are required to work with beams in the range up to 100 milliamps and to implant high mass ions (such as arsenic). Such high current ion beams must be space charge neutralized by electrons traveling with the beam to avoid beam expansion, or blowup, due to charge repulsion. The use of electrostatic deflection elements with high beam currents results in the removal of the neutralizing electrons and an unacceptable beam expansion due to space charge repulsion. Magnetic elements do not remove electrons from the neutralized beam, but for high atomic mass ions, magnetic deflection elements are large, heavy and power consuming. A magnetic double deflection system for use in an ion implanter is disclosed in U.S. Pat. No. 4,367,411.
Ion beam lithography utilizes a finely focused ion beam which is deflected over the surface of a workpiece. Scanning with a constant angle of incidence is desirable in the case of ion beam lithography for the same reasons as set forth above in connection with ion implantation.
It is a general object of the present invention to provide novel ion beam scanning apparatus.
It is another object of the present invention to provide a high current ion implantation system with a constant angle of incidence between the ion beam and the workpiece.
It is still another object of the present invention to provide ion implantation apparatus wherein a space charge lens is utilized to achieve a constant angle of incidence between the ion beam and the workpiece.